Naked‐Eye Visual Warning of ppm _v ‐Level Fault‐Free Humidity Upper Limit

Monitoring and warning of ppm_v-level fault-free humidity upper limit (ppm_v-FFHUL) are crucial for ensuring system reliability in critical fields such as high-tech manufacturing, and aerospace. However, existing ppm_v-FFHUL warning technologies are scarce, and constrained by "gradual change" response modes, resulting in complicated equipment, high costs, and insufficient portability, which greatly limits their flexible application. To address this, we propose a "leap change" response mode and an "enzyme-like" construction method for precise ppm_v-level humidity threshold modulation. It is achieved through synergistic supramolecular interactions between two size-matched auxiliary mediums to precisely construct an "enzyme like" microenvironment containing a matching number of multi-level, spatially fixed sensitizing functional groups around an elaborately designed humidity-sensitive component. The as-prepared material combines paper-like flexibility, facile fabrication and handling, and enables high-contrast readout of the target ppm_v-FFHUL only by naked-eye. Moreover, it holds promise as an easily deployable ppm_v-FFHUL warning labels and provide a visual pre-alert signal 20% ahead of the critical value. It offers a portable and low-cost solution for specialized scenarios that require early warning and rapid screening of ppm_v-level humidity. Furthermore, the "enzyme-like" precise construction method and the "leap change" response mode provide an innovative perspective for the design of other high-performance trace-level monitoring materials.